(1) Field of the Invention
The present invention relates to a sealed battery, especially to a sealed battery having a closure cap with a gas release valve that covers the opening of the external casing.
(2) Related Art/Description of the Conventional Art
Recently, sealed batteries have been used as the power source of portable electric appliances such as mobile phones, audiovisual devices, and computers.
As typical sealed batteries, alkaline batteries such as nickel-metal hydride batteries and nickel-cadmium batteries, and lithium-ion batteries are used. Usually, these sealed batteries are cylinder or rectangular-cylinder ones. Among them, rectangular-cylinder sealed batteries receive attention due to a space-saving advantage. More specifically, the shape of a rectangular-cylinder battery can be easily determined according to the inner space of a portable electric appliance.
An explanation of the construction of a sealed battery will be given below. The external casing of metal cylinder encloses a generator element. The generator element is an electrode that is composed of positive and negative electrodes impregnated with electrolyte. The opening of the external casing is covered by a closure cap. In order to prevent leakage of the electrolyte or gas, the closure cap is sealed. For this sealing, although mechanical caulking is adopted in many cases, laser welding is often adopted for rectangular-cylinder sealed batteries.
Meanwhile, in order to increase the energy density, the generator element and the closure cap are set fairly close to each other so that the generator element shares most of the internal space of the external casing. The external casing has been conventionally made of nickel-plated steel or stainless steel. Recently, however, light-weight batteries using an aluminum alloy for the external casing have been under development.
For the sealed battery, the closure cap or the external casing is provided with a gas release valve for releasing gas when the internal pressure of the battery has been increased to a certain level.
As the gas release valve for sealed batteries, a resilient valve is often used for alkaline batteries. For instance, a resilient valve that covers the gas release hole in the closure cap or the external casing with a valve plug using a spring and the like is often adopted. On the other hand, a non-resilient valve is often used for nonaqueous electrolyte batteries since high hermeticity is required. For instance, a non-resilient valve that covers the gas release hole in the metal closure cap by attaching a thin film (rupture) so as to cover the gas release hole as described in Japanese Laid-Open Patent Application No. 6-68861.
For the safety of portable electric appliances, the electrolyte should not leak even if sealed batteries are dropped.
In drop test, however, a problem is often found for sealed batteries with the closure cap provided with the non-resilient gas release valve. The thin film of the gas release valve is broken and the electrolyte leaks.